This invention relates to a polyurethane binder and to processes for producing a low-viscosity polyurethane binder containing isocyanate groups which only has a low content of readily volatile residual monomers, essentially forms no xe2x80x9cmigratesxe2x80x9d and shows improved lubricant adhesion. The invention also relates to the use of a low-viscosity polyurethane binder containing isocyanate groups (NCO groups) in the production of adhesives, more particularly one-component and two-component adhesives, for example for bonding web-form materials of, for example, paper, plastic and/or aluminium, coatings, more particularly lacquers, emulsion paints and casting resins as well as moldings.
Isocyanate-terminated polyurethane prepolymers have been known for some time. They may readily be chain-extended or crosslinked with suitable compounds, usually polyhydric alcohols, to form high molecular weight materials. Polyurethane prepolymers have acquired significance in many fields of application, including for example the production of adhesives, coatings, casting resins and moldings.
In order to obtain isocyanate-terminated polyurethane prepolymers, it is standard practice to react polyhydric alcohols with an excess of polyisocyanates, generally at least predominantly diisocyanates. Molecular weight can be controlled at least approximately through the ratio of OH groups to isocyanate groups. Whereas a ratio of OH groups to isocyanate groups of, or approaching, 1:1 leads to generally high molecular weights, a statistical average of one diisocyanate moleculexe2x80x94where diisocyanates are usedxe2x80x94is attached to each OH group where the OH: isocyanate group ratio is about 2:1, so that ideally no oligomerization or chain extension occurs in the course of the reaction.
In practice, however, chain-extending reactions are impossible to suppress completely, even in the last case mentioned, with the result that, on completion of the reaction, a certain quantity of the component used in excess is left over irrespective of the reaction time. If diisocyanate, for example, is used as the excess component, a generally considerable proportion of this component remains behind in the reaction mixture for the reasons explained above.
The presence of such components is particularly problematical when they consist of readily volatile diisocyanates. The vapors of these diisocyanates are often harmful to the skin and the application of products with a high content of such readily volatile diisocyanates requires elaborate measures on the part of the user to protect the people involved in processing the product, more particularly elaborate measures for keeping the surrounding air clean to breathe.
Since protective measures and cleaning measures generally involve considerable expense, there is a need on the part of the user for products which have a low percentage content of readily volatile diisocyanates depending on the isocyanate used.
In the context of the present invention, xe2x80x9creadily volatilexe2x80x9d substances are understood to be substances which have a vapor pressure at around 30xc2x0 C. of more than about 0.0007 mmHg or a boiling point of less than about 190xc2x0 C. (70 mPa).
If low-volatility diisocyanates, more particularly the widely used bicyclic diisocyanates, for example diphenyl methane diisocyanates, are used instead of the readily volatile diisocyanates, polyurethane binders with a viscosity normally outside the range suitable for simple processing methods are generally obtained. In cases such as these, the viscosity of the polyurethane prepolymers can be reduced by adding suitable solvents although this is not consistent with the absence of solvents normally demanded. Another way of reducing viscosity without solvents is to add an excess of monomeric polyisocyanates which are incorporated in the coating or bond (reactive diluent) in the course of a subsequent curing/hardening process (after the addition of a hardener or by curing under the influence of moisture).
Whereas the viscosity of the polyurethane prepolymers can actually be reduced in this way, the generally incomplete reaction of the reactive diluent often leads to the presence in the bond or coating of free monomeric polyisocyanates which are capable of xe2x80x9cmigratingxe2x80x9d, for example within the coating or bond or, in some cases, even into the coated or bonded materials themselves. Corresponding constituents of a coating or bond are often referred to among experts as xe2x80x9cmigratesxe2x80x9d. By contact with moisture, the isocyanate groups of the migrates are continuously reacted to form amino groups. The aromatic amines normally formed in this way are suspected of having a carcinogenic effect.
Migrates are often not tolerable, above all in the packaging field, because any migration of the migrates through the packaging material would result in contamination of the packaged product and the consumer would inevitably come into contact with the migrates when using the product.
Accordingly, the migrates in question are undesirable above all in the packaging field, especially in the packaging of foods.
In order to avoid the disadvantages described above, EP-A 0 118 065 proposes producing polyurethane prepolymers by a two-stage process. In the first stage of this process, a monocyclic diisocyanate is reacted with a polyhydric alcohol in an OH group:isocyanate group ratio of  less than 1 and, in the second step, a bicyclic diisocyanate is reacted with polyhydric alcohols in an OH group:isocyanate group ratio of  less than 1 in the presence of the prepolymer prepared in the first step. A ratio of OH groups to isocyanate groups of 0.65 to 0.8:1 and preferably 0.7 to 0.75:1 is proposed for the second stage. The prepolymers obtainable in this way still have viscosities of 2500 mPas, 7150 mPas and 9260 mPas at high temperatures (75xc2x0 C. and 90xc2x0 C.). Lubricant compatibility is not mentioned.
EP-A 0 019 120 relates to a two-stage process for the production of elastic weather-resistant sheet-form materials. In the first stage of this process, toluene diisocyanate (TDI) is reacted with at least equimolar quantities of a polyol and the reaction product obtained is subsequently reacted with diphenyl methane diisocyanate (MDI) and a polyol. The polyurethane binders obtainable in this way are said to be capable of curing with water or with atmospheric moisture. Although the described process does give products with a relatively low viscosity, the content of free readily volatile diisocyanate (in the present case TDI) is still high (0.7% by weight) and can only be reduced when time-consuming and energy-intensive methods, for example thin-layer distillation, are used to remove excess readily volatile diisocyanate.
Hitherto unpublished German patent application DE 197 49 834.5 relates to low-monomer polyurethane binders, disclosing a two-component polyurethane binder with a low monomer content and a low migrate content.
Films used, for example, for the packaging of foods frequently have a high content of lubricants. A typical lubricant is, for example, erucic acid amide (EAA) which is often present in the film in quantities of more than about 400 ppm. Conventional adhesives used for bonding such films to form laminates often show a deterioration in laminate adhesion with increasing lubricant content of the film to be bonded. With modern efficient packaging machines, an increase in productivity is achieved by considerably increasing the operating speed of the machines. The films used for this purpose generally have a distinctly increased lubricant content xe2x80x94often of 600 ppm or morexe2x80x94in relation to relatively slow packaging machines. Unfortunately, many conventional adhesives show unsatisfactory adhesion on films with a lubricant content of this order. As a result, laminated films thus produced often show inadequate sealing seam adhesion, above all after curing.
Accordingly, the problem addressed by the present invention was to provide a polyurethane binder which would have a low viscosity and a low residual content of less than about 1% by weight of readily volatile diisocyanates. In the case of toluene diisocyanate (TDI), the residual content of readily volatile isocyanate should be less than about 0.1% by weight.
A further problem addressed by the present invention was to provide a polyurethane binder which would have a low percentage content of xe2x80x9cmigratesxe2x80x9d, i.e. a low percentage content of monomeric polyisocyanates.
Another problem addressed by the present invention was to provide a process for the production of a polyurethane binder having the properties mentioned above.
Yet another problem addressed by the invention was to provide a polyurethane binder and an adhesive which, besides the above-mentioned properties in regard to its content of readily volatile isocyanates and a low migrate content, would additionally have improved lubricant compatibility, i.e. for example improved laminate adhesion or improved sealing seam adhesion in the bonding of films with a high lubricant content.
The present invention relates to a polyurethane binder with a low content of readily volatile isocyanate-functional monomers at least containing components A, B and C, in which
a) a polyurethane polymer containing at least two isocyanate groups or a mixture of two or more polyurethane prepolymers containing at least two isocyanate groups is present as component A, at least one polyurethane prepolymer containing at least two differently attached types of isocyanate groups or two different polyurethane prepolymers containing at least two differently attached types of isocyanate groups in pairs of which at least one type has a lower reactivity to isocyanate-reactive groups than the other type(s), and
b) an at least difunctional isocyanate which does not contain one nitrogen atom that is not part of an NCO group, or a mixture of two or more thereof, is present as component B and
c) an at least difunctional isocyanate or a mixture of two or more isocyanates with an average functionality of at least two which contains at least one nitrogen atom that is not part of an NCO or urethane group per isocyanate molecule is present as component C.